The analysis of particular lethal mutations has demonstrated the existence of neoplasms in Drosophila melanogaster. We have isolated and begun the analysis of temperature-sensitive alleles at two neoplastic loci, Tum1 and I(2)gl. The Tum1 mutation develops a neoplasm of the larval lymph gland and the 1(2)gl-ts mutation develops a neoplasm of the wing imaginal disc. We have demonstrated that the expression of the Tum mutation is associated with the overproduction of differentiated hemocytes both in situ during larval development and during the culture of the mutant lymph gland in adult hosts. Current research involves the analysis of the relationship between the overproduction of differentiated hemocytes and the initiation of neoplastic development, as well as the effect of parasitism on the expression of the mutation. We have demonstrated that the 1(2)gl-ts wing discs become neoplastic when either 1(2)gl-ts embryos or developing larvae are exposed to 29~C. In addition, the nature 1(2)gl-ts wing discs become neoplastic during culture in adult hosts at 29~C, but not at 15~C. However, amputated 1(2)gl-ts wing discs become neoplastic during culture at 15~C. Since neither 1(2)gl-ts wing discs that have been cut, but not amputated, and wing discs that have received 1,800 rads of gamma radiation do not become neoplastic, the amputation-induced neoplastic development of the 1(2)gl-ts wing discs is not simply a result of trauma. Current research on the 1(2)gl-ts mutation involves the analysis of the relationship between the induction of regeneration and the promotion of neoplastic development, the temperature and culture conditions that result in the initiation of neoplastic development of these discs, and the changes in the architecture, proliferation and the developmental capacity of the tissue that occurs during the initiation of neoplastic development.